Fuji alters pixel structure for CCD breakthrough

TOKYO  Fuji Photo Film Co. Ltd. has developed a prototype charged-coupled device that sports an array of honeycomb-shaped photodiodes, and which the company said is better than conventional digital camera CCDs by just about any performance measure. The device has the potential to singlehandedly expand the range of digital imaging products, the company said.

The new CCD structure, which was developed in collaboration with Fujifilm Microdevices Co. Ltd., a Fuji subsidiary, is expected to be employed in a new line of sub-$1,000 consumer digital cameras that the company will bring to market sometime next year. Fuji said it plans to eventually sell the so-called Super CCD devices on the merchant market  a first for Fujifilm, said Yasuo Nagashima, assistant manager of technical service for the electronic imaging products division at Fuji Photo Film (Tokyo).

The Super CCD chip, with its larger, more densely-packed photodiode structure, is an improvement over today's CCDs in areas such as resolution, color reproduction, signal-to-noise, dynamic range, sensitivity and power consumption, according to the company.

The device used the human eye as a model for how to receive optical information, company officials said. To mimic the eye, Fuji tilted to a 45 angle the CCD's tiled pixel structures containing the light-sensitive photodiode and its attached electrodes. In this way, the vertical and horizontal spacing between the photodiodes becomes smaller than diagonal intervals. In conventional CCDs, the opposite is true, the company said.

In addition, Fujifilm created a larger octoganal-shaped photodiode so that more of the CCD area is used for soaking up incoming photons, further enhancing horizontal/vertical resolution. A larger photodiode also helps offset loss of sensitivity for each photodiode when the device undergoes a process technology shrink, which is done to cram more pixels on a device. For Fujifilm's 2-million-pixel, 0.5-inch CCD prototype, the pixel pitch is 4.5 microns, Nagashima said. He declined to describe the process technology used to make the part.

Because the octagonal shape is closer to the circular microlens that sits on top of the pixel, light can be collected more efficiently, the company said. And with the new photodiode structure, Fujifilm was able to further maximize the light-sensitive area of the CCD by eliminating the control signal path, so that the pixel now only contains one large photodiode and a charge transmission path around its periphery.

For comparison, Fuji Photo said a 1.9-million pixel CCD based on this structure will have an equivalent resolution of a 3-million-pixel CCD using standard rectangular pixels. And Fujifilm claims that the aggregate photodiode charge in a 0.5-inch Super CCD will continue to increase as the pixel count goes up, rather than staying constant, as with conventional CCDs.

The company also boasts greater than twofold improvements in signal-to-noise, dynamic range and sensitivity, though the company did not provide actual specifications. And because a Super CCD doesn't need as many pixels as a regular CCD to provide the same resolution, digital camera OEMs have an opportunity to reduce power consumption.

The company expects Super CCD to expand the range of functions and products using CCDs. Better sensitivity will enable faster shutter speeds and longer flash range. The simpler two-poly structure, four-packet photodiode and wide charge transmission path allows the pixels to be read in one step rather than the normal two, so no mechanical shutter is needed.

The part's smaller size in comparison to other CCDs with similar pixel counts opens the door for the design of ultra-compact digital cameras. The CCD can also be used for full-motion video. "This technology may be applied to a movie camera, and any conventional CCD might be replaced using this CCD," Nagashima said.

One challenge Fuji faces is making the device production-worthy, Nagashima said. "Conventional CCDs use a vertical and horizontal layout, but to make this work we need to have diagonal lines, so in that sense it might be a bit more difficult to manufacture. But it has only two poly levels so in this sense it is simpler," he said.

By next year, the company expects to introduce a digital camera using a Super CCD with more than 3 million pixels, Nagashima said. And that's just the start. "Theoretically, we can make CCDs with more than 10 million pixels based on this technology," he said.